PET imaging involves the creation of tomographic images of positron emitting radionuclides in a subject of interest. A radionuclide-labeled pharmaceutical, i.e., a radiopharmaceutical, is administered to an imaging subject. The subject is positioned within a PET imaging system which includes a detector ring and detection electronics. As the radionuclides decay, positively charged photons known as “positrons” are emitted. For commonly used radiopharmaceuticals such as FDG, (i.e., 18F-fluorodeoxyglucose), these positrons travel only a few millimeters through the tissues of the subject before colliding with an electron, resulting in mutual annihilation. The positron/electron annihilation results in a pair of oppositely-directed gamma rays that are emitted with approximately 511 keV energy.
It is these gamma rays that are detected by the scintillator components of the detector ring. When struck by a gamma ray, the scintillating material in these components emits light, which is detected by a photodetector component, such as a photodiode or photomultiplier tube. The signals from the photodetectors are processed as incidences of gamma rays. When two gamma rays strike oppositely positioned scintillators at approximately the same time, a coincidence is registered. Data sorting units process the coincidences to determine which are true coincidence events and sort out data representing dead times and single gamma ray detections. The coincidence events are binned and integrated to form frames of PET data which may be reconstructed as images depicting the distribution of the radionuclide-labeled pharmaceutical in the subject.
MRI is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis”, by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when current is applied to them. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. These coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system, and combined with multiple additional such signals may be used to reconstruct an MR image using a computer and known algorithms.
Combining PET and MRI in a single scanner presents difficult technical challenges. An MRI scanner is typically designed to have the gradient coils, RF coils, shielding and cooling systems packed as close together as possible. Prior combined systems have located the PET detector components outside of an RF shield and within the gradient coil and magnet space of the MR magnet assembly. For example, prior solutions have included splitting the gradient coil to make space for a ring of PET detectors, splitting the gradient coil and magnet to make space for a ring of PET detectors or within the gradient coil and magnet space, separating the crystal and detector electronics used in the PET detector with fiber optic cables. However, these arrangements can take up significant radial space. In addition, the PET and MRI systems must not interfere with one another electrically. Accordingly, it would be desirable to provide a solution for integrating a PET detector, for example a ring of PET detectors, into a MRI magnet assembly by integrating the PET detector with the RF hardware components of the magnet assembly.